Allergic Reactions

How an allergic reaction happens

An allergic reaction takes more than one encounter with the allergen. The first exposure produces no reaction at all; what it does is prompt the immune system to build antibodies specific to that allergen. On the second exposure and on every one after it, those waiting antibodies bind the allergen, and that binding releases histamine and other chemicals into the body.

Those chemicals set off the allergic response as a cascade that strikes several systems at once. The blood vessels dilate, which lowers cardiac output and puts the patient at risk of shock. The capillaries leak, producing angioedema, the swelling of the face, lips, tongue and airway. The bronchioles constrict and fill with mucus, narrowing the airways and producing wheezing. The skin breaks out in hives and itching.

The allergens responsible fall into a few familiar groups. Insects, particularly bees and wasps, cause reactions that are rapid and severe. Foods are the next group, and peanuts are especially severe and especially rapid. Then come plants such as poison ivy, medications such as penicillin, and latex. A severe reaction usually begins immediately, but it can be delayed by 30 minutes or more, and a reaction that begins mild can escalate into anaphylaxis within minutes.

Distinguishing anaphylaxis from a mild reaction

What separates a life-threatening reaction from an uncomfortable one is not how distressed the patient looks but which systems the reaction has reached. A reaction is anaphylaxis when it produces either respiratory distress or signs and symptoms of shock. Either finding alone is enough; you do not wait for both before you treat. The table below compares the two across the systems you assess.

Table 23.1Distinguishing anaphylaxis from a mild reaction

Treatment & epinephrine

Any patient in respiratory distress, and every patient who is anaphylactic, receives high-concentration oxygen (O2) by nonrebreather mask (NRB). For anaphylaxis, though, oxygen is not the intervention that decides the outcome. Prompt epinephrine is the single most important lifesaving measure available to you.

The decision to give it rests on two things: signs of anaphylaxis — respiratory distress, swelling of the face or airway, or signs of shock — and access to the drug, either because the patient has a prescribed auto-injector of their own or because your protocol allows the EMT to carry epinephrine. A history of earlier reactions supports the picture, but a first-ever reaction is treated the same way. When both are present, administer the epinephrine in accordance with medical direction. When no auto-injector is available and your protocol does not allow EMT-carried epinephrine, treat the patient for shock, transport immediately, and request an advanced life support (ALS) intercept.

Self-administered epinephrine works on two fronts at once: it constricts the blood vessels, which raises blood pressure (BP), and it dilates the bronchioles, which improves breathing. To give it, check the expiration date, remove the cap, and press the injector firmly against the outer thigh, midway between the waist and the knee, holding it there until the dose is fully injected. Reassess the patient 2 minutes later.

Pediatric note & additional doses

The auto-injector comes in two sizes. The adult injector delivers 0.3 mg of epinephrine, and the pediatric injector, used for a child weighing less than 66 lbs (30 kg), delivers 0.15 mg. Anaphylaxis does occur in infants, and it is missed more often than it is absent. The signs are harder to read at that age — hives, swelling, persistent vomiting, coughing or wheezing, mottling and inconsolable crying — so the reaction goes unrecognized and epinephrine is under-used. Age is never a reason to withhold it.

A second dose is sometimes needed, and giving one depends on two things: having additional auto-injectors on hand, and obtaining authorization from medical direction for the repeat dose.